What is the significance of a 'SG' prefix on a Singapore private hire car's vehicle plate?

1. Standard Singapore-registered private hire cars use standard red-and-white plates; there is no specific 'SG' prefix requirement — LTA does not require a special PHC plate designation
2. PHC vehicles must display 'SG PHC' plates in yellow
3. All private hire cars must have green-coloured number plates
4. PHC vehicles use blue plates similar to government vehicles